The present invention relates to electromotive devices and more particularly to an ironless core armature for an electric motor.
Electric motor manufacturers and in particular DC motor manufacturers have traditionally employed wire winding or printed circuit coil techniques to fabricate ironless core armatures, which move in a magnetic flux air gap. There, however, are a number of problems associated with these designs. Ironless core motors are typically run with a larger gap than conventional iron core designs. The iron core motors have wire wound through a core of magnetically permeable material and the iron core is cut to minimize the gap but iron core motors have more mass in the armature than ironless core motors.
In the wire winding case, the insulated wire is wrapped in a multilayer configuration to form the current carrying coil with a specific conductor to insulation volume ratio known as packing density. With typical circular coil wire, the insulation material and air voids inherent in this coil construction make for a less than optimal conductor packing density. If square or rectangular conductors are used for armature winding, both the packing density of the coil as well as the total volume of conductor within the magnetic gap are increased. Coil wire is usually circular wire which consists of an electrical conductor (copper or aluminum) surrounded by an insulation layer on top of which there is a bonding layer for structural stability. In most prior art armature wire windings of this type, the conductor packing density is about 60%. If square wire is used in traditional armature production instead of circular wire, the conductor packing density is increased to 70%-80%. Manufacturers, however, prefer using circular wire due to its lower material and labor cost and ease of manufacturing. Therefore, a need exists for a new armature design that is cost effective to produce and that would result in a higher conductor packing density as well as a higher volume of conductor in the magnetic gap. Some ironless core armatures are wire wrapped in angular fashion allowing conductor to conductor bonding for ease of manufacturing and structural integrity which is less efficient because electron flow should be at 90 degrees to the magnetic flux path for maximum efficiency. Angular wrapped armatures exhibit reduced torque by the sine of the angle of the current to the magnetic field. The structure of wire wrapped armatures makes it difficult to produce long small diameter armatures with adequate strength to withstand the destructive centrifugal forces of high RPM applications.
Armatures built by Printed Circuit manufacturing techniques involve rotor windings being formed as flexible printed circuits. Printed circuits are circuits in which the conducting material is applied to an insulated support base by adhesives and etched from one side. The amount of electrical conductor in this case is compromised, however, as multiple layers of insulated printed circuit traces tend to result in a thicker armature wall and a diminished conductor packing density. The packing density of this type of armature is lowered due to the volume of flexible printed circuit insulation material used to support the conductive loops during fabrication. Reducing the armature wall thickness with thin wraps of printed circuit traces tends to weaken armature walls and yield higher electrical resistance due to narrower and thinner conductor traces. Higher electrical resistance results in an undesirable increase in motor heat and energy dissipation, thus causing power losses equal to P=I2R. Alternatively, wider printed circuit traces improve motor performance by reducing trace electrical resistance, but allow eddy currents, which reduce the overall gain by again increasing the effective electrical resistance. Printed circuit construction can be found in larger gap motors where multiple layers are used to create multiple turn coils, to increase the length of conductor in the magnetic field. This results in a thicker armature structure and a larger magnetic gap. These flexible circuits are mostly used in brushless motor applications where the windings are held stationary and the magnet is rotated. The larger number of windings creates an armature of larger inductance and higher electrical resistance.
Various attempts have been made in the prior art to improve ironless core armature performance. For example, U.S. Pat. No. 3,944,857 to Faulhaber discloses an air-core or ironless core armature for electrodynamic machines having an elongated insulating strip rolled up to form a spiral structure composed of a number of radially successive layers. An armature winding is comprised of at least one armature coil and each coil is comprised of a number of electrically interconnected component coils. Each coil is formed of electrically interconnected conductor sections printed on both sides of the insulating strip. This set up, unfortunately, does not optimize the configuration of the windings so as to produce optimal torque.
U.S. Pat. No. 3,805,104 to Margrain is directed to a hollow insulating cylinder with conductors which are placed over an internal metallic tubular support which is supported by an end disk at one end, and open at the other end, the open end being flared for stiffness. The cylinder has insulation with the electrical conductors being in printed or laminated circuit form. This type of device, however, compromises the conductor packing density factor and does not produce optimal torque.
The Lorentz Law for Electromotive Devices is F=Ixc3x97Lxc3x97B; where F=Force, I=current, L=conductor length, B=magnetic flux density. The Lorentz Law theory as it applies to electric motors is clearly illustrated in FIGS. 10a, b and c. FIG. 10a illustrates the environment we see in traditional wire wound armatures in use today. Wire wound conductors must have wire insulation which decreases the carrier packing density and thereby the current density per unit area, and thereby the inability to uniformly mount the armature in such manner to cut a maximum of flux lines. In addition, wire wound armatures must be wound at an angle thereby creating a angle between the crossed vectors of Current and Magnetic Flux that is less than the maximum desired ninety degrees to yield the greatest force.
FIG. 10b illustrates the metal strip carriers envisioned by an embodiment of the invention disclosed herein. It can be observed that a square cross section of FIG. 10b will enable a greater proximity of the flat sided current carrier to the means from which the magnetic flux emanates/terminates in the gap between current flow/conductor and said means where the greatest flux density exists. The round cross section of a conventional wire wound armature does not permit such close proximity of the current carrier and the magnetic field carrier. In addition, the square cross section can be increased to a rectangular cross section as indicated in FIG. 10b to yield an even greater current density and flow in a very much reduced magnetic flux gap where the flux density is at its greatest.
Incorporating the complete current loop illustrated in FIG. 10c, it becomes very apparent that the doubled Lorentz Force resulting from the same force on each arm of the conductor and imposed on the flat conductor surface of FIG. 10b will be substantially increased by the increased current density, increased flux density and a maximum ninety degree angle between the current and the flux. This is the substantial factor in the Lorentz Force equation. Conventional wire wound armatures are disposed at an angle to the Magnetic Flux Density; therefore, the Current (I) vector/flow is at an angle to the Magnetic Flux (B) vector which of necessity yields a smaller resulting Lorentz Force (F).
The vector diagrams of FIGS. 10a, b and c clearly illustrate that the force (therefore torque) on the armature of the type described above can be increased by optimizing or increasing each of the terms of the equation. In particular, current (I) flow may be maximized by reducing the electrical resistance of the conductor coil. In a specific armature design, conductor length (L) increases can be obtained from multiple conductor loops. Magnetic flux density (B) can be maximized by minimizing the air gap between the magnetic return structure and the magnet wherein the armature rotates, that is, the closer the magnetic return path is to the magnet, the higher the magnitude of the magnetic flux density (B) will be. For any given current in the windings of the armature, which is placed in the higher magnetic field, torque will be increased. Therefore, to improve efficiency of the traditional motor, an armature manufacturer or designer should strive to reduce the armature wall thickness which will result in the ability to reduce the size of the magnetic gap creating higher flux density, this should be accomplished without sacrificing conductor length or increasing conductor resistance. Positioning the armature wall in close proximity to the magnetic field origin and return path will allow for more conductor volume for a given gap width, less electrical resistance and result in an increase in the conductor density within the gap.
In one aspect of the present invention, a method of fabricating an inductive coil from a pair of conductive sheets includes forming in each of the conductive sheets a plurality of conductive bands each being separated from an adjacent band by a space, shaping the conductive sheets into inner and outer winding portions, positioning the inner winding portion within the outer winding portion, and coupling each of the conductive bands of the inner winding portion to one of the conductive bands of the outer winding portion, electrically insulating a thermally conductive flywheel, and assembling the flywheel to the coil.
In another aspect of the present invention, a method of fabricating an inductive coil from a pair of conductive sheets includes cutting each of the conductive sheets into a pattern comprising a plurality of conductive bands each being separated from an adjacent band by a space, rolling the conductive sheets into inner and outer cylindrical winding portions, telescoping the inner winding portion into the outer winding portion, and coupling each of the conductive bands of the inner winding portion to one of the conductive bands of the outer winding portion.
It is understood that other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only embodiments of the invention by way of illustration of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive.